1. Field of the Invention
This invention relates generally to a fluid sampling tool and method of use which, in response to pressure, opens to collect a fluid sample, and more particularly, to a sampling tool which provides for collection of a fluid sample without flashing of vapor in the liquid and which retains the fluid in a supercharged condition.
2. Description of the Prior Art
In general, to obtain a sample of fluid in an oil or gas well, a fluid sampling tool is first lowered into the well on a tubing string or a wireline or a slick line. When the tool is at the desired depth, a port (one or more openings) defined in the tool is opened. The port may open in response to pressure exerted through the well fluid or in response to an electrical actuation signal from the surface. The open port admits well fluid into a sample retaining chamber within the tool. The port is thereafter closed, the tool is withdrawn from a well, and the sample is taken from the chamber for analysis.
U.S. Pat. No. 4,903,765 to Zunkel, assigned to the assignee of the present invention, shows an improvement in such fluid sampling tools, wherein the fluid sampling tool is constructed to have a time delay which starts when a valve of a tool first starts to move in response to pressure from the well. This time delay provides various advantages. In one instance, the time delay allows undesired fluid such as drilling fluids to bypass the sampling tool before the valve communicates a sample port with a sample chamber and a sample of the well fluid is taken. In another instance, the time delay can reduce the dependency on accurate pressure readings and shear pins which control the opening of the valve. For example, when a maximum bottom hole pressure is measured or otherwise anticipated, shear pins providing a holding force of something less than this maximum pressure, but one which will clearly be encountered somewhere downhole despite a lack of assurance as to precisely where it will be, can be used so that the pins will break at some location above the bottom of the well. This time delay, designed with a suitable tolerance to assure reaching bottom before its expiration, is then used to allow the tool to be run on down to the well bottom, where it is ultimately automatically opened.
U.S. Pat. No. 5,058,674 to Schultz et al., also assigned to the assignee of the present invention, provides various improvements upon a delayed opening fluid sampler of the type generally shown in the Zunkel patent. These improvements relate generally to various means for controlling the actuation of the valve which controls flow of the sample fluid to the sample chamber.
A problem with some prior art fluid samplers is that the sample is obtained relatively quickly which can cause the fluid to flash (separation of the liquid and vapor stages) as it is flowing into the sampling chamber. This is an undesirable condition and can affect the quality of the fluid sample. The sampler of the present invention provides for controlled flowing of the fluid into the sample chamber which greatly reduces or eliminates fluid flashing.
Another problem with some prior fluid samplers is that when they are removed from the wellbore, the reduction in hydrostatic pressure acting on the sampler as it is raised also results in fluid pressure therein being reduced. The drop in pressure can cause phase change degradation of the sample. That is, flashing can occur as the sampler is removed from the wellbore. The sampler of the present invention solves this problem by providing for the fluid sample to be trapped at well hydrostatic pressure regardless of the pressure outside the sampler. This "supercharging" of the fluid sample greatly reduces or eliminates phase change problems.